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I've pasted the content of /usr/share/grub/grub-mkconfig_lib underneath (pastebin was deleted as apparent from comment below). You can download it and place it in appropriate place, then retry.

Also, look for the file /usr/lib/grub/grub-mkconfig_lib. Generally, it's a symbolic link to /usr/share/grub/grub-mkconfig_lib. But do check if the original file is there. If in your case /usr/lib/grub/grub-mkconfig_lib is the original file then open the file /etc/grub.d/10_linux and change its 21st line from datarootdir="${prefix}/share" to datarootdir="${prefix}/lib" then complete the upgrade.